4. Kangaroos are good runners that can sustain a speed of 56 km/h (15.5 m/s).
Suppose a kangaroo is sitting on a log that is floating in a lake. When the
kangaroo gets scared, she jumps off the log with a velocity of 15 m/s toward the
bank. The log moves with a velocity of 3.8 m/s away from the bank. If the mass
of the log is 250 kg, what is the mass of the kangaroo?